Fuel compositions

ABSTRACT

A fuel composition containing a gas oil base fuel and a C 4-8  alkyl levulinate. The alkyl levulinate is selected for the purpose of ensuring a phase separation temperature of said fuel composition below a predetermined level.

The present invention relates to fuel compositions comprising a gas oil base fuel, particularly to such compositions containing a levulinate ester and to their preparation and use.

Two different fuel components can be blended together so as to modify the properties and/or the performance, e.g. engine performance, of the resultant composition.

Diesel fuel components include the so-called “biofuels” which derive from biological materials. Examples include levulinate esters.

Levulinate esters (esters of levulinic acid) and their preparation by reaction of the appropriate alcohol with furfuryl acetate are described in Zh. Prikl. Khim. (Leningrad) (1969) 42(4), 958-9, and in particular the methyl, ethyl, propyl, butyl, pentyl and hexyl esters.

WO-A-94/21753 discloses fuels for internal combustion engines, including both gasoline and diesel fuel, containing proportions (e.g. 1 to 90% v, 1 to 50% v, preferably 1 to 20% v) of esters of C₄₋₆ keto-carbonic acids, preferably levulinic acid, with C₁₋₂₂ alcohols. Esters with C₁₋₈ alcohols are described as being particularly suitable for inclusion in gasolines, and esters with C₉₋₂₂ alcohols are described as being particularly suitable for inclusion in diesel fuels.

The examples in WO-A-94/21753 are all of the inclusion of quantities of levulinate esters in gasolines, for improvement in octane numbers (RON and MON).

WO-A-03/002696 discloses a fuel composition incorporating levulinic acid, or a functional derivative thereof, with the object of providing more oxygen by volume than ethanol or traditional oxygenates such as MTBE or ETBE, giving little or no increase in fuel Reid vapour pressure and little or no effect on the flash point of the base fuel. The functional derivative is preferably an alkyl derivative, more preferably a C₁₋₁₀ alkyl derivative. Ethyl levulinate is said to be preferred, with methyl levulinate a preferred alternative. The levulinic acid or functional derivative is preferably used to form 0.1 to 5% v of the fuel.

Whilst WO-A-03/002696 states (page 11, line 31) that “The foregoing is illustrated by the following examples”, the compositional and test result data consists of the following sentences:

-   -   “Specification gasoline blends containing up to 5.0% ethyl         levulinate, 1.0% water and 2.0% non-ionic surfactant were found         to have similar RVPs to the base gasoline.”, and “Specification         diesel blends containing up to 5.0% ethyl levulinate, 1.0% water         and 2.0% non-ionic surfactant were found to have similar flash         points to the base diesel.”

Current commercially available compression ignition (diesel) engines tend to be optimised to run on fuels having a desired specification. Moreover, the conditions under which the engine is required to operate can affect the manner in which a fuel composition in the engine will behave. In particular, as the atmospheric temperature falls, the miscibility between components in the fuel composition will deteriorate. Such a deterioration in miscibility manifests itself as an increase in the phase separation temperature, which is defined as the temperature at which, on cooling, the mixture separates into distinct immiscible layers. The blending of a standard commercial diesel base fuel with other fuel components, to modify the overall fuel properties and/or performance, can therefore have an adverse impact on the performance of the blend in the engines for which it is intended.

A further complication can arise when an engine is run on a fuel blend instead of a standard base fuel. Within the engine fuel injection system, the fuel comes into contact with a range of elastomeric materials, in particular fuel pump seals. In use, many of these elastomers swell on contact with diesel fuel to an extent which depends on the chemistry of the fuel, aromatic fuel components and oxygenates serving for instance to promote swelling.

New elastomers in a fuel injection system tend to equilibrate with a uniform fuel diet and can thus provide with reasonable consistency the required level of sealing. They become vulnerable, however, if a change in fuel diet causes any significant change in the degree of elastomer swell. In the worst cases a mixed fuel diet can stress the elastomeric components of an engine to such an extent that fuel leakage results.

For the above reasons, it is desirable for any diesel fuel blend to have an overall specification as close as possible to that of the standard commercially available diesel base fuels for which engines tend to be optimised.

This can, however, be difficult to achieve because any additional fuel component is likely to alter the properties and performance of the base fuel. Moreover the properties of a blend, in particular its effect on elastomeric engine components and on low temperature performance, are not always straightforward to predict from the properties of the constituent fuels alone.

It has now been found that in fuel compositions comprising a gas oil base fuel and an alkyl levulinate, selection of said alkyl levulinate from C₄₋₈ alkyl levulinates ensures that the phase separation temperature of the fuel composition is below a predetermined level. It has also been found that said fuel compositions containing C₄₋₈ alkyl levulinates are more compatible with certain elastomeric seal materials than such fuel compositions containing similar concentrations of ethyl levulinate, the compatibility being not significantly different from that of the base fuel.

Accordingly, in one embodiment, a fuel composition is provided comprising a fuel composition comprising a gas oil base fuel and an alkyl levulinate, said alkyl levulinate is a C₄₋₈ alkyl levulinate.

For example, it has been found that if 5% v of ethyl levulinate blended into certain base fuels is replaced by 5% v of certain C₄₋₈ alkyl levulinates, the phase separation temperature is greatly reduced, i.e. improving the miscibility between the base fuel and the levulinate. This can of course be extremely advantageous when the fuel blend is for use in an engine operating in a low temperature environment. Moreover, it has been found that the blends containing C₄₋₈ alkyl levulinates have substantially less effects on elastomer swell and hardness than the blends containing ethyl levulinate.

In one embodiment of the present invention there is provided a fuel composition comprising a gas oil base fuel and an alkyl levulinate, wherein the alkyl levulinate is a C₄₋₈ alkyl levulinate. Preferably, the alkyl levulinate is selected from C₄₋₈ alkyl levulinates, such as n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate and 2-ethyl hexyl levulinate, for the purpose of ensuring a phase separation temperature of the fuel composition below a predetermined level. The level preferably is −10° C., more preferably −20° C., and most preferably −30° C. Preferably, the alkyl levulinate is selected from C₄₋₆ alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or the alkyl levulinate is a C₅ alkyl levulinate. In the fuel composition, the alkyl levulinate preferably is n-pentyl levulinate.

In one embodiment of the invention there is provided use in a fuel composition comprising a gas oil base fuel, an alkyl levulinate of a C₄₋₈ alkyl levulinate as the alkyl levulinate, for the purpose of ensuring a phase separation temperature of the fuel composition below a predetermined level. The alkyl levulinate is selected from C₄₋₈ alkyl levulinate effective to produce a phase separation temperature of the fuel composition below a predetermined level. The level preferably is −10° C., more preferably −20° C., and most preferably −30° C. Preferably, in the use the alkyl levulinate is selected from C₄₋₆ alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or the alkyl levulinate is a C₅ alkyl levulinate. The alkyl levulinate preferably is n-pentyl levulinate.

According to the present invention, there is further provided a method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and ethyl levulinate, which method comprises replacing at least partially the ethyl levulinate with a C₄₋₈ alkyl levulinate. The method preferably comprises reducing the phase separation temperature below a predetermined level, the level preferably being −10° C., more preferably −20° C., and most preferably −30° C.

According to the present invention there is still further provided a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition according to the present invention.

According to the present invention there is yet further provided a method of operating a heating appliance provided with a burner, which method comprises supplying to the burner a fuel composition according to the present invention.

According to the present invention, there is yet further provided a process for the preparation of a fuel composition which process involves blending a gas oil base fuel and a C₄₋₈ alkyl levulinate. Preferably, in the process the alkyl levulinate is selected from C₄₋₆ alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or the alkyl levulinate is a C₅ alkyl levulinate. In the process, the alkyl levulinate preferably is n-pentyl levulinate.

In all aspects of the present invention, blends of two or more of the C₄₋₈ alkyl levulinates may be included in the fuel composition, such as for example a blend n-butyl levulinate and n-pentyl levulinate. In the context of the present invention, selection of the particular components of the blends and their proportions is dependent upon one or more desired characteristics of the fuel composition.

The present invention may be used to formulate fuel blends which are expected to be of particular use in modern commercially available diesel engines as alternatives to the standard diesel base fuels, for instance as commercial and legislative pressures favour the use of increasing quantities of organically derived “biofuels”.

In the context of the present invention, “use” of a fuel component in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more other fuel components, conveniently before the composition is introduced into an engine.

The fuel composition will typically contain a major proportion of the base fuel, such as from 50 to 99% v, preferably from 50 to 98% v, more preferably from 80 to 98% v, most preferably from 90 to 98% v. The proportions of the C₄₋₈ alkyl levulinates will be chosen to achieve the desired degree of miscibility, i.e. phase separation temperature, and elastomer swell and hardness effects, and may also be influenced by other properties required of the overall composition.

The effects on elastomeric engine components may include changes in the physical properties (e.g. volume, hardness and/or flexibility) of a given elastomeric material on contact with, suitably immersion in, the relevant fuel or fuel composition, for instance inside a diesel engine into which the relevant fuel is introduced. Tyically such changes include an increase in volume and/or a reduction in hardness. They may be measured using standard test procedures such as BS903, ASTM D471, D2240 or ISO 1817:1998, for instance as described in Example 2 below. They may be assessed in particular for nitrile (including hydrogenated nitrile) elastomers, or for fluorocarbon elastomers.

Preferably the C₄₋₈ alkyl levulinates are included in the fuel composition at proportions such as to cause a change in volume of any given elastomeric material (for example a fluorocarbon type such as LR 6316 (ex. James Walker & Co. Ltd., UK)) which is not significantly different from that caused by the base fuel when tested under the same conditions.

Preferably the C₄₋₈ alkyl levulinates are included in the fuel composition at proportions such as to cause a change in hardness of any given elastomeric material (for example a fluorocarbon type such as LR 6316) which is not significantly different from that caused by the base fuel when tested under the same conditions. Yet more preferably, the proportions are such as to achieve a change in elastomer hardness which is no higher than that of the base fuel alone, ideally 95% or 90 % or 85% or less of that caused by the base fuel.

The fuel compositions to which the present invention relates include diesel fuels for use in automotive compression ignition engines, as well as in other types of engine such as for example marine, railroad and stationary engines, and industrial gas oils for use in heating applications (e.g. boilers).

The base fuel may itself comprise a mixture of two or more different diesel fuel components, and/or be additivated as described below.

Such diesel fuels will contain a base fuel which may typically comprise liquid hydrocarbon middle distillate gas oil(s), for instance petroleum derived gas oils. Such fuels will typically have boiling points with the usual diesel range of 150 to 400° C., depending on grade and use. They will typically have a density from 750 to 900 kg/m³, preferably from 800 to 860 kg/m³, at 15° C. (e.g. ASTM D4502 or IP 365) and a cetane number (ASTM D613) of from 35 to 80, more preferably from 40 to 75. They will typically have an initial boiling point in the range 150 to 230° C. and a final boiling point in the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTM D445) might suitably be from 1.5 to 4.5 mm²/s.

Such industrial gas oils will contain a base fuel which may comprise fuel fractions such as the kerosene or gas oil fractions obtained in traditional refinery processes, which upgrade crude petroleum feedstock to useful products. Preferably such fractions contain components having carbon numbers in the range 5-40, more preferably 5-31, yet more preferably 6-25, most preferably 9-25, and such fractions have a density at 15° C. of 650-950 kg/m³, a kinematic viscosity at 20° C. of 1-80 mm²/s, and a boiling range of 150-400° C.

Optionally, non-mineral oil based fuels, such as biofuels or Fischer-Tropsch derived fuels, may also form or be present in the fuel composition.

The amount of Fischer-Tropsch derived fuel used in a diesel fuel composition may be from 0.5 to 100% v of the overall diesel fuel composition, preferably from 5 to 75% v. It may be desirable for the composition to contain 10% v or greater, more preferably 20% v or greater, still more preferably 30% v or greater, of the Fischer-Tropsch derived fuel. It is particularly preferred for the composition to contain 30 to 75% v, and particularly 30 or 70% v, of the Fischer-Tropsch derived fuel. The balance of the fuel composition is made up of one or more other fuels.

An industrial gas oil composition will preferably comprise more than 50 wt %, more preferably more than 70 wt %, of a Fischer-Tropsch derived fuel component.

Such a Fischer-Tropsch derived fuel component is any fraction of the middle distillate fuel range, which can be isolated from the (hydrocracked) Fischer-Tropsch synthesis product. Typical fractions will boil in the naphtha, kerosene or gas oil range. Preferably, a Fischer-Tropsch product boiling in the kerosene or gas oil range is used because these products are easier to handle in for example domestic environments. Such products will suitably comprise a fraction larger than 90 wt % which boils between 160 and 400° C., preferably to about 370° C. Examples of Fischer-Tropsch derived kerosene and gas oils are described in EP-A-0583836, WO-A-97/14768, WO-A-97/14769, WO-A-00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83648, WO-A-01/83647, WO-A-01/83641, WO-A-00/20535, WO-A-00/20534, EP-A-1101813, U.S. Pat. No. 5,766,274, U.S. Pat. No. 5,378,348, U.S. Pat. No. 5,888,376 and U.S. Pat. No. 6,204,426, the disclosures are hereby incorporated by reference.

The Fischer-Tropsch product will suitably contain more than 80 wt % and more suitably more than 95 wt % iso and normal paraffins and less than 1 wt % aromatics, the balance being naphthenics compounds. The content of sulphur and nitrogen will be very low and normally below the detection limits for such compounds. For this reason the sulphur content of a fuel composition containing a Fischer-Tropsch product may be very low.

The fuel composition preferably contains no more than 5000 ppmw sulphur, more preferably no more than 500 ppmw, or no more than 350 ppmw, or no more than 150 ppmw, or no more than 100 pp 0 mw, or no more than 50 ppmw, or most preferably no more than 10 ppmw sulphur.

In addition to the C₄₋₈ alkyl levulinates, the fuel composition of the present invention may, if required, contain one or more additives as described below.

The base fuel may itself be additivated (additive-containing) or unadditivated (additive-free). If additivated, e.g. at the refinery, it will contain minor amounts of one or more additives selected for example from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers) and wax anti-settling agents (e.g. those commercially available under the Trade Marks “PARAFLOW” (e.g. PARAFLOW™ 450, ex Infineum), “OCTEL” (e.g. OCTEL™ W 5000, ex Octel) and “DODIFLOW” (e.g. DODIFLOW™ v 3958, ex Hoechst).

Detergent-containing diesel fuel additives are known and commercially available, for instance from Infineum (e.g. F7661 and F7685) and Octel (e.g. OMA 4130D). Such additives may be added to diesel fuels at relatively low levels (their “standard” treat rates providing typically less than 100 ppmw active matter detergent in the overall additivated fuel composition) intended merely to reduce or slow the build up of engine deposits.

Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene) maleic anhydrides. Succinimide dispersant additives are described for example in GB-A-960493, EP-A-0147240, EP-A-0482253, EP-A-0613938, EP-A-0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides.

The additive may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers such as those commercially available as NALCO™ EC5462A (formerly 7D07) (ex Nalco) and TOLAD™ 2683 (ex Petrolite); anti-foaming agents (e.g. the polyether-modified polysiloxanes commercially available as TEGOPREN™ 5851 and Q 25907 (ex Dow Corning), SAG™ TP-325 (ex OSi) and RHODORSIL™ (ex Rhone Poulenc)); ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in U.S. Pat. No. 4,208,190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. that sold commercially by Rhein Chemie, Mannheim, Germany as “RC 4801”, a propane-1,2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid); corrosion inhibitors; reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,N′-di-sec-butyl-p-phenylenediamine); and metal deactivators.

It is particularly preferred that the additive include a lubricity enhancer, especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration between 50 and 1000 ppmw, preferably between 100 and 1000 ppmw. Suitable commercially available lubricity enhancers include EC 832 and PARADYNE™ 655 (ex Infineum), HITEC™ E580 (ex Ethyl Corporation), VEKTRON™ 6010 (ex Infineum) and amide-based additives such as those available from the Lubrizol Chemical Company, for instance LZ 539 C. Other lubricity enhancers are described in the patent literature, in particular in connection with their use in low sulphur content diesel fuels, for example in:

-   -   the paper by Danping Wei and H. A. Spikes, “The Lubricity of         Diesel Fuels”, Wear, III (1986) 217-235;     -   WO-A-95/33805—cold flow improvers to enhance lubricity of low         sulphur fuels;     -   WO-A-94/17160—certain esters of a carboxylic acid and an alcohol         wherein the acid has from 2 to 50 carbon atoms and the alcohol         has 1 or more carbon atoms, particularly glycerol monooleate and         di-isodecyl adipate, as fuel additives for wear reduction in a         diesel engine injection system;     -   U.S. Pat. No. 5,484,462—mentions dimerised linoleic acid as a         commercially available lubricity agent for low sulphur diesel         fuel (column 1, line 38), and itself provides         aminoalkylmorpholines as fuel lubricity improvers;     -   U.S. Pat. No. 5,490,864—certain dithiophosphoric         diester-dialcohols as anti-wear lubricity additives for low         sulphur diesel fuels; and     -   WO-A-98/01516—certain alkyl aromatic compounds having at least         one carboxyl group attached to their aromatic nuclei, to confer         anti-wear lubricity effects particularly in low sulphur diesel         fuels.

It is also preferred that the additive contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity additive.

Unless otherwise stated, the (active matter) concentration of each such additional component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range from 5 to 1000 ppmw, advantageously from 75 to 300 ppmw, such as from 95 to 150 ppmw.

The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 600 ppmw or less, more preferably 500 ppmw or less, conveniently from 300 to 500 ppmw.

If desired, the additive components, as listed above, may be co-mixed, preferably together with suitable diluent(s), in an additive concentrate, and the additive concentrate may be dispersed into the fuel, in suitable quantity to result in a composition of the present invention.

In the case of a diesel fuel, for example, the additive will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a carrier oil (e.g. a mineral oil), a polyether, which may be capped or uncapped, a non-polar solvent such as toluene, xylene, white spirits and those sold by member companies of the Royal Dutch/Shell Group under the trade mark “SHELLSOL”, and/or a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by member companies of the Royal Dutch/Shell Group under the trade mark “LINEVOL”, especially LINEVOL™ 79 alcohol which is a mixture of C₇₋₉ primary alcohols, or the C₁₂₋₁₄ alcohol mixture commercially available from Sidobre Sinnova, France under the trade mark “SIPOL”.

The total content of the additives may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.

Preferably, the C₄₋₈ alkyl levulinate concentration in the fuel composition accords with one or more of the following parameters:

-   -   (i) at least 1% v; (ii) at least 2% v; (iii) at least 3% v; (iv)         at least 4% v; (v) up to 6% v; (vi) up to 8% v; (vii) up to 10%         v, (viii) up to 12% v,         with ranges having features (i) and (viii), (ii) and         (vii), (iii) and (vi), and (iv) and (v) respectively being         progressively more preferred.

In this specification, amounts (concentrations, % v, ppmw, wt %) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.

The present invention is particularly applicable where the fuel composition is used or intended to be used in a direct injection diesel engine, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or in an indirect injection diesel engine. It may be of particular value for rotary pump engines, and in other diesel engines which rely on mechanical actuation of the fuel injectors and/or a low pressure pilot injection system. The fuel composition may be suitable for use in heavy and/or light duty diesel engines.

It is also applicable where the fuel composition is used in heating applications, such as boilers, including standard boilers, low temperature boilers and condensing boilers. Such boilers are typically used for heating water for commercial or domestic applications such as space heating and water heating.

The present invention may lead to any of a number of advantageous effects, including good engine low temperature performance.

The present invention will now be described by reference to the non-limiting following Examples:

Fuels were blended with additives by adding additive to base fuel at ambient temperature (20° C.) and homogenising.

The following additives were used:

-   -   ethyl levulinate (available ex. Avocado);     -   n-butyl levulinate (available ex. Aldrich);     -   n-pentyl levulinate (available ex. City Chemical or by the         reaction of 1-pentanol (available ex. Aldrich) with levulinic         acid (available ex. Aldrich);     -   2-hexyl levulinate (prepared by the reaction of 1-hexene         (available ex. Fluka) or of 2-hexanol (available ex. Aldrich)         with levulinic acid).

EXAMPLE 1

Miscibility of Alkyl Levulinates in Diesel Fuel (AGO)

The miscibility of levulinates depends to some extent on base fuel properties. Three base fuels representative of the European market were chosen to explore this effect, i.e. (1) Fuel A was an ultra low sulphur diesel (ULSD), which is typical of a 2005 specification European diesel fuel, with a cloud point of −8° C. and an aromatics content of 25% m; (2) Fuel B was a Dreyfuss ULSD, which is a hydrotreated AGO having a lower cloud point (−27° C.) and a similar aromatics content to Fuel A (22% m), which complied with European specification EN590; and (3) Fuel C was a Swedish Class 1 AGO, which is a low density, low aromatics (4% m) diesel fuel with the lowest cloud point of the three base fuels (−38° C.).

The properties of Fuels A, B and C are given in Table 1. TABLE 1 Fuel A Fuel B Fuel C Density @ 15° C., 834 822 815 kg/m³ Distillation T50, 280 242 235 ° C. Distillation T95, 343 304 272 ° C. Cetane Number 56 54 54 Viscosity @ 40° C., 2.91 2.10 2.03 mm²/s Sulphur, mg/kg 38 10 <5 Cloud Point, ° C. −8 −27 −38 Aromatics, % m 25 22 4

For screening purposes, a simple test method was used to determine the room temperature (20° C.) limit of miscibility of ethyl levulinate. Accurately metered volumes of ester were added sequentially to a known volume of diesel fuel in a 15 ml glass vial, shaken and observed. The first appearance of haze was recorded as the room temperature limit of miscibility for the mixture. The results are shown in Table 2 and clearly show that Fuel C was the most severe of the three base fuels tested. This fuel was selected for further miscibility testing. TABLE 2 Fuel A Fuel B Fuel C 10% v 14% v 7% v

The miscibility of various alkyl levulinates was measured using a method based on the ASTM D2500 “Cloud Point” procedure. In this procedure, a sample of fuel (40 ml) is cooled from ambient temperature (20° C.) in a series of thermostat baths maintained at progressively lower temperatures. The sample is examined at 1° C. intervals as it cools to its wax cloud point. In addition to the wax cloud point temperature described in ASTM D2500, a further two temperatures were recorded coinciding with the following observations, if they occurred:

-   -   (1) the appearance of the first haze,     -   (2) the first sign of dropout of a separate liquid phase.

In each case, cooling continued to the wax cloud point—beyond which, no further phase separation could be observed reliably.

Solutions of ethyl levulinate, n-butyl levulinate and n-pentyl levulinate in Fuel C were blended at various concentrations and the miscibility of each blend was measured. The results are shown in Table 3 below. TABLE 3 Ester Phase separation temperature (° C.) concentration ethyl n-butyl n-pentyl (% v) levulinate levulinate levulinate 1 −37 — — 2 −26 −38 −38 3 −10  −37*  −38* 4 3 −36 −38 5 5  −31*  −38* 6 — −26 −38 8 — −22 −33 10 — −18 −28 *extrapolated values

It can be seen from Table 3 that both n-butyl levulinate and n-pentyl levulinate had superior miscibility in Fuel C to ethyl levulinate. For example, at 5% v of ethyl levulinate, the phase separation temperature was 5° C., whilst at 5% v of n-butyl levulinate or n-pentyl levulinate, the phase separation temperatures were below −30° C. It is to be noted that concentrations of up to between 8 and 10% v of n-butyl levulinate and up to at least 10% v of n-pentyl levulinate remained in solution at temperatures below −20° C., even in this severe Swedish Class 1 AGO.

The miscibility tests were repeated using Fuel B to confirm this finding in a more conventional European EN590 specification diesel fuel. These results are shown in Table 4. TABLE 4 Ester Phase separation temperature (° C.) concentration ethyl n-butyl n-pentyl (% v) levulinate levulinate levulinate 2 — −28 −27 3 −27  −27*  −27* 4 −17 −27 −27 5 −10  −27*  −27* 6  −5* −28 −27 8    7 −27 −26 10   14 −27 −28 *extrapolated values

It can be seen from Table 4 that both n-butyl levulinate and n-pentyl levulinate had superior miscibility in Fuel B to ethyl levulinate at concentrations of 4% v and above. For example, at 5% v of ethyl levulinate, the phase separation temperature was −10° C., whilst at 5% v of n-butyl levulinate or n-pentyl levulinate, the phase separation temperatures were both −27° C. It is to be noted that concentrations of up to at least 10% v of n-butyl levulinate and n-pentyl levulinate remained in solution at temperatures below −20° C., and the wax cloud points were reached before phase separation was observed.

EXAMPLE 2

Effect of Alkyl Levulinates on Fluorocarbon Elastomer Swell

The effect of various alkyl levulinate compounds on elastomer seals was assessed using a test procedure based on ISO 1817:1998. The volume and average Shore hardness of elastomer samples, nominally 50 mm×25 mm×3 mm thickness, were measured both before and after immersion in 10 ml of test fuel at ambient temperature (20° C.) for 168 hours. Thereafter, the samples were removed from the test fluid, quickly surface dried, weighed in air and in water and their new volume and hardness measured within 8 hours of their removal from the test medium. Hardness was measured at ambient temperature using a Type A Shore™ Durometer (Shore Instruments, USA). The percentage changes in volume and in average hardness, due to exposure to the test fuel, were then reported for each sample.

Tests were conducted to compare the effects on elastomers of: ethyl levulinate, n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate. Each of these compounds was blended at 5% v concentration into a base fuel, Fuel D, which was a conventional diesel fuel sample. The properties of Fuel D and of a blend of 5% v n-pentyl levulinate in Fuel D are shown in Table 5. TABLE 5 5% v n-pentyl EN 590:2000 levulinate Property spec. Fuel D in Fuel D Density @15° C., kg/m³ 820-845 834.2 841.1 Distillation IBP 179.7 185.0 10% 215.2 217.0 20% 236.7 234.5 30% 254.0 250.0 40% 268.6 264.0 50% 280.3 276.5 60% 290.4 288.5 70% 300.4 299.5 80% 311.6 311.5 90% 326.7 328.0 95% 360 max 338.9 343.5 FBP 353.1 352.0 Rec at 240° C., % v 22.4 23.5 Rec at 250° C., % v  65 max 27.6 30.0 Rec at 340° C., % v 95.3 94.0 Rec at 345° C., % v 96.7 95.5 Rec at 350° C., % v  85 min 97.9 96.5 Cetane number  51 min 55.2/54.8 53.4 Viscosity @40° C.,   2-4.5 2.910 2.884 mm²/s Sulphur, mg/kg 350 38 — Lubricity <460 302/298 — (HFRR wear scar, μm) Flash point, ° C. >55 67 74.5 Peroxide content, Report 0.5 0.8 ppm “Rec” = “recovered”

The elastomer material was chosen to be representative of the seals (O-rings, etc.) used in modern diesel fuel systems: LR 6316 (a fluorocarbon tetrapolymer also known as Viton (trade mark) (ex. James Walker & Co. Ltd., UK). It was chosen as an elastomer which is typical of those used in modern diesel fuel systems and which, although less susceptible to seal swell than some other elastomer materials, is able to highlight significant changes in swell properties.

The effect of the various levulinate blends on the volume and hardness of LR 6316 fluorocarbon elastomer samples is summarised in Table 6. TABLE 6 % v % Volume % Hardness Component/Blend oxygenate change change Fuel D 0 0.02 −1.3 ethyl levulinate 5 10.63 −14.4 n-butyl 5 2.4 −0.4 levulinate n-pentyl 5 1.7 −0.83 levulinate 2-hexyl 5 1 0 levulinate

It can be seen that n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate produced substantially less seal swell (i.e. % volume change) than ethyl levulinate, and that the change in hardness with n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate was substantially less than with ethyl levulinate and not significantly different from conventional Fuel D.

The ISO 1817 standard explicitly states that “no direct correlation with service behaviour is implied”, so no “pass/fail” threshold can be defined without reference to the final application. However, if it were to be considered that fuels or fuel additives showing a seal swell of 2% or less with LR 6316 fluorocarbon elastomer are unlikely to cause problems in service, then it can be seen from Table 6 that n-pentyl levulinate and 2-hexyl levulinate would be preferred levulinate esters. 

1. A fuel composition comprising a gas oil base fuel and an alkyl levulinate, said alkyl levulinate is a C₄₋₈ alkyl levulinate.
 2. The fuel composition of claim 1 wherein said alkyl levulinate is selected from C₄₋₈ alkyl levulinates effective to produce a phase separation temperature of said fuel composition below a predetermined level.
 3. The fuel composition of claim 2 wherein said level is −10° C.
 4. The fuel composition of claim 2 wherein said level is −20° C.
 5. The fuel composition of claim 2 wherein said level is −30° C.
 6. The fuel composition of claim 1 wherein said alkyl levulinate is selected from C₄₋₆ alkyl levulinates.
 7. The fuel composition of claim 6 wherein the alkyl levulinate is selected from the group consisting of n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate, and mixtures thereof.
 8. The fuel composition of claim 6 wherein said alkyl levulinate is selected from C₄₋₈ alkyl levulinates effective to produce a phase separation temperature of said fuel composition below a predetermined level.
 9. The fuel composition of claim 8 wherein said level is −10° C.
 10. The fuel composition of claim 8 wherein said level is −20° C.
 11. The fuel composition of claim 8 wherein said level is −30° C.
 12. A method of reducing the phase separation temperature of a fuel composition comprising a gas oil base fuel and ethyl levulinate, which method comprises replacing at least partially said ethyl levulinate with a C₄₋₈ alkyl levulinate.
 13. The method of claim 12 which comprises reducing the phase separation temperature below a predetermined level of −10° C.
 14. The method of claim 12 wherein said level is −20° C.
 15. The method of claim 12 wherein said level is 30° C.
 16. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising introducing into a combustion chamber of the engine a fuel composition of claim
 1. 17. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising introducing into a combustion chamber of the engine a fuel composition of claim
 3. 18. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine comprising introducing into a combustion chamber of the engine a fuel composition of claim
 6. 19. A method of operating a heating appliance provided with a burner comprising supplying to said burner a fuel composition of claim
 1. 20. A method of operating a heating appliance provided with a burner comprising supplying to said burner a fuel composition of claim
 3. 21. A method of operating a heating appliance provided with a burner comprising supplying to said burner a fuel composition of claim
 6. 22. A process for the preparation of a fuel composition comprising blending a gas oil base fuel and a C₄₋₈ alkyl levulinate.
 23. The process of claim 22 wherein the alkyl levulinate is selected from C₄₋₆ alkyl levulinates.
 24. The process of claim 23 wherein the alkyl levulinate is selected from the group consisting of n-butyl levulinate, n-pentyl levulinate, 2-hexyl levulinate and mixtures thereof. 